Non-impact offset printers using an electrically charged wire to transfer toner to a paper surface are known. A dielectric belt is provided with a conductive coating underneath the dielectric material and is arranged in a tensioned endless loop. The charge on the belt is selectively modified by a print head to attract toner to create a latent image. Movement of the belt around its loop transfers the image to a so-called wrap area or toner transfer zone in which the belt is held in intimate contact with a paper surface. The toner transfer zone is exposed to an electric field of a wire called a transfer corona which causes an electric charge to accumulate in the paper. Toner in the form of an image remains on the paper after separation from the dielectric belt and is subsequently fixed onto the paper such as by application of beat at a fusing station. The belt later is cleaned of residual toner and electrically conditioned for re-use at the print head.
Attraction of toner to paper by the transfer corona requires that the paper surface be moving in synchronism with the belt and in direct contact with the latent image on the belt during exposure to the electric field of the transfer corona. If the electric field builds up on the paper in the area before the toner transfer zone, such premature exposure of paper to the corona prior to paper contact with the toner and belt causes toner to lift from the belt in an unconfined cloud.
Depending upon the length of such premature exposure, the sharpness of the latent image on the belt may be lost prior to exposure in the toner transfer zone where the belt and paper are in contact. The consequence is a loss of quality of the printed image on the paper.
Known approaches to controlling exposure of the latent image on the belt have been largely directed to the use of an extension of a corona shell to block the electric field of the corona wire in the non-contact area. However, operation of the printer desirably requires lifting paper from the belt in the toner transfer zone so that the paper can be stopped at the end of a printing cycle while the image belt continues to move. Known corona shell extensions are fixed relative to the corona wire, and a gap consequently exists between the corona shell extension and belt at a point prior to the entrance of the belt to the toner transfer zone. Therefore, the possiblity of premature exposure of paper to the corona remains, and image quality is degraded by any unintended transfer of toner prior to contact of the belt and toner with the paper surface.